A speed controller of an electric motor may be used for not only making a following action to a speed command but also making a high-speed positioning action by adding a position controlling loop outside a speed loop. It is essential that the speed controlling system is able to follow a speed command at a high speed and high accuracy. Hereinafter, prior arts pertaining to the abovementioned field of art are listed, and a description is given of features in each case.
(1) Motor speed controller which feeds back only speed signals resulting from a position differential
In this art, if a proportional gain of the speed loop is increased to achieve a high speed response, high harmonic ripple components in the detecting speed is accordingly amplified to cause the torque wave form to be distorted, whereby such a problem occurs, where motor noise and vibrations are increased. Where an electric motor driven by the motor speed controller actuates a mechanism, distortion of the abovementioned torque waveform will become a source of vibration with respect to resonance elements of the mechanism and power transmitting system, or the existence of resonance characteristics may damage margin in stability of the control loop, whereby resonance noise occurs when driving the mechanism and the control system may be made unstable. Therefore, since the size of the proportional gain of the motor speed controller is limited due to distortion of the torque waveform and decrease in the allowance in stability of the controlling loop, such a problem occurs, by which the response characteristics of the speed-controlling system cannot be improved.
(2) Method for passing a speed-detected output through a low-pass filter
A method for passing a speed-detected output through a low-pass filter to decrease noise is considered in order to smoothen the torque waveform, wherein since the allowance in stability of the control loop is damaged by an increase in phase delay, the proportional gain of the speed-controlling system cannot be increased, and the response characteristics cannot be improved.
(3) Method for applying a same-dimensions (status) observer
It is considered that a same-dimensions (status) observer is concurrently applied in the speed-controlling system in order to smoothen the torque waveform and secure allowance in stability of the controlling system. Since the same-dimensions (status) observer is a prediction type, it can be easily imagined that speed signals having less phase delay can be outputted, and the speed signals can be achieved.
(4) Method for controlling vibrations by using an equivalent rigid body observer
Japanese Patent Application No. 12785 of 1994 (Laid-Open Patent Publication No. 337057 of 1995) and Japanese Patent Application No. 233528 of 1995 (Laid-Open Patent Publication No. 56183 of 1997) exist as a means for increasing the proportional gain of the speed-controlling system, which drives a mechanism system influenced by mechanical resonance elements, with allowance in stability of the control loop secured. These are such that a control structure consisting of an equivalent rigid body observer, low-pass filter, high-pass filter and vibration controlling gain is added to the speed-controlling system.
Shortcomings and problems pertaining to the present invention are summarized as follows;
(a) Generally, even though a speed-controlling system is of a proportional plus integral type since a constant deviation occurs in detected signals due to disturbance such as Coulomb friction in the status observer, there is a problem in that the output of the speed controlling system is not coincident with the command speed. Therefore, usual status observers cannot be applied to the speed-controlling system as it is.
(b) In a motor speed controller using an equivalent rigid body observer, there are a number of parameters to be adjusted. It is necessary to add and adjust at least three parameters in total, such as a observer gain, time constants of low-pass filter and high-pass filter, and vibration controlling gain in addition to normal control parameters of the speed controlling system.
(c) There is no adjustment procedure for high-speed responses in the speed controlling system, or if any, the adjustment procedure is complicated. Thus, in the prior art, there are a number of parameters to be adjusted, whereby the adjustment procedure is complicated, and a problem occurs, in which no adjustment procedure is provided.
As a means for solving the above, by taking a larger observer gain than the gain of the speed controlling system, setting is determined so that the dynamic characteristics of the observer do not influence the dynamic characteristics of the speed controlling system, wherein there are many cases where adjustment of the controlling system is simplified.
(d) The speed controlling system, in which an observer is incorporated, is not constructed so that the response characteristics thereof can be easily improved. That is, in this case, since the cut-off frequency of the observer (as a filter) is high, the mechanical resonance components cannot be sufficiently reduced by the observer, whereby the proportional gain of the speed controlling system could not be increased (A detailed description is given later).
Based on the above consideration, it is therefore an object of the invention to provide a motor speed controller having a control structure by which the number of parameters to be adjusted may be reduced, and the proportional gain of the speed controlling system is increased, in a speed controlling system in which an equivalent rigid body observer (observer) is incorporated, and steps for adjusting so as to easily accelerate the speed response in the controlling system.